Extracorporeal circulation of blood is used in certain surgical procedures such as during heart surgery. During the extracorporeal circulation, the body temperature of the patient can be controlled, by controlling the temperature of the blood during extracorporeal circulation. For this purpose, a patient temperature control system can be provided by means of which the temperature of the blood of the patient in the circulation can be raised or lowered. The blood thus controlled, flows through the patient and the body of the patient approaches the temperature of the blood. So as to heat or cool the blood, the temperature control system comprises a heater and/or cooler device providing a liquid circulation to a disposable (single use) heat exchanger that transfers energy to and/or away from the patient's blood circulation. The liquid can be water. The heat exchanger for the blood is a strict dual circuit system/ the blood side and the liquid side being separated from each other so that any mixture, such as by means of diffusion, between the blood in one of the circuits and the temperature control liquid in the other of the circuits is inhibited as much as possible. Nevertheless, care has to be taken to avoid health risks stemming from the liquid.